1. Technical Field
The present invention relates to an electrodynamic-transducer magnetic motor device with a moving coil, of the type comprising a magnetic circuit consisting of an annular bonded magnet.
The invention is in particular intended to form part of an electrodynamic loudspeaker. However, the invention may be applied to any type of magnetic motor with a moving coil.
2. Related Art
An electrodynamic loudspeaker magnetic motor with a moving coil comprising a magnetic circuit formed by a bonded magnet is already known from the example provided in the patent document in the name of the Applicants published under the reference number WO2009/133149. According to this document, the conventional loudspeaker motor with permanent magnets and iron plates at the front and the rear for guiding the field lines is replaced by a bonded magnet structure in the form of a ring made of plasto-magnetic material (thermoplastic bonding material) or elasto-magnetic material (elastomer bonding material). The bonded magnets are in fact made by means of injection in a mold which may have a very large variety of forms. It is thus possible to create parts where the useful magnetic field is improved and consequently limit the leakage field which is the main defect of conventional sintered magnets.
Thus the document WO2009/133149 relates to a magnetic motor device without field plates, but where the permanent magnet is an annular bonded magnet with a particular form having a cylindrical surface and, opposite thereto, a convex surface. This document discloses in particular a magnetic device where the bonded magnet is installed inside the moving coil support, the bonded magnet having an outer cylindrical surface which extends facing the wire windings of the coil and a convex surface which extends towards the inside of the magnet. This convex surface is such that the form traced by an axial plane of the bonded magnet and the convex surface is a hemi-ellipse or a semicircle. Moreover, the outer cylindrical surface has two cylindrical parts opposite each other in relation to the mid-plane of the magnet.
In this way, along an axial plane, the field lines extend, from one side to the other inside the magnet, parallel to the curvature defined by the hemi-elliptical convex surface and intersecting substantially perpendicularly the cylindrical surface. This allows the magnetic field to be concentrated effectively towards the wire windings of the coiled support.
However, the field lines do not close up easily beyond the coiled support, opposite the magnet. Thus, the document WO2009/133149 discloses the installation of a second bonded magnet around the coil support and symmetrical in relation to the one which is housed inside so as to close up the field lines and obtain a more linear magnetic field and limit the magnetic leakages.
However, the installation of an additional magnet around the coiled support increases the weight and the volume of the magnetic motor device. In this respect, in order to achieve better integration of the electrodynamic transducer, a reduction in the magnetic mass is particularly desirable.
Moreover, simulations of the magnetic field generated inside a bonded-magnet annular structure with a semicircular or hemi-ellipsoidal form as described in the document WO2009/133149 have led to the conclusion that this form is not optimum in terms of useful magnetic field. FIG. 1 shows in this connection an example of calculation of the magnetic field obtained in a bonded-magnet annular motor 30 according to the document WO2009/133149, having an inner cylindrical surface and, opposite thereto, a convex surface which extends towards the outside of the magnet and the convex surface of which is such that the intersection of an axial plane of the bonded magnet and the convex surface is semicircular, as schematically shown in the cross section alongside the graph of FIG. 1. This bonded magnet is intended to surround the coil support so that the inner cylindrical surface extends facing the wire windings of the coil. The graph in FIG. 1 shows an example of a magnetic field expressed in Tesla (T) obtained inside this annular motor at a constant distance from the cylindrical surface, as a function of the height z expressed in millimeters of the magnet structure in relation to a mid-plane P of the structure, perpendicular to the axis of revolution Z of the magnet. The hatched zone in the graph corresponds to a zone in the center of the material of the annular motor where the magnetic field is weak or difficult to control during industrial production of the magnet.